**Introduction**

The study of physics has captured the human mind for centuries. Encompassing some of the greatest minds of history, the evolution of physics has followed the human race since the beginnings of civilization. Filled with both complementing and contrasting thought experiments, theories, and revelations, the journey to modern-day physics is an exciting and complicated journey. The present article aims to provide an overview of the journey of physics and the main highlights of the journey. The major focus is on quantum mechanics, and the problems and theories of modern physics it has led to. The article discusses some thought experiments relating to quantum mechanics, how quantum mechanics changed the Newtonian view of the world, and some other interesting topics in modern physics.

**Determinism and Uncertainty**

From the beginning, physics was driven by the spirit of mechanism. Natural phenomena have mechanical causes. Then the idea emerged that there were particles like electrons on one hand, and fields like gravitational and electromagnetic fields on the other. With the birth of quantum mechanics, physics turned toward a more unified worldview. Both particles and forces are manifestations of a deeper level of reality – the level of quantum fields. In quantum field theory, all particles are bundles of energy of their respective fields.

It all began with Sir Issac Newton. While the concept of gravity was discovered ages before him, and his formula no longer holds for some physical cases, it was he who dared to think, so long back in a conservative society, that just like a rock thrown with enough velocity, the moon might be constantly “falling”, relative to the earth. He dared to call his law “universal”, to think beyond and apply it to all celestial bodies, not just the Earth and the Sun.

Though the distribution of matter (like stars etc.) in the universe does not seem homogeneous, the universe can be assumed to be isotropic and homogeneous, when it is considered *in its entirety*. This is why the gravitational pull on a star is almost equal from all directions and thus, the star can exist at a point in a stable state (this is how Newton tried to solve Bentley’s paradox, which stated that all stars and celestial bodies should implode into a point due to the attracting nature of gravity [1]). These hypotheses form the basis of cosmology. Though the Earth cannot be concluded to be exactly at the center of the universe, due to the ever-expanding nature of the universe, it appears that the universe is approximately isotropic, i.e., it looks more or less the same in all directions.

The world was satisfied by the Newtonian explanation of the universe. Some people, like French scholar Laplace, were seized by the dream of an absolutely certain universe. Laplace believed that, if the positions and momenta of all the particles of the universe are known, then, in principle, it becomes possible to predict the states of these particles at any time, in the past or future. However, Heisenberg’s uncertainty principle[2] has proved that it is impossible to determine, with a hundred percent certainty, both the position and momenta of even a single particle, leave alone all the particles of the universe. Also, both the errors in energy and time cannot be simultaneously known for a particle.

Einstein tried to challenge the uncertainty principle. He imagined a box of light. Theoretically, a single photon could be released from the box by a clockwork shutter. Thus, the exact time could be recorded. Also, the box could be weighed before and after the photon is released. Finding the change in mass, using , the energy of the photon could also be known.

Einstein found an apparent violation of the uncertainty principle because we seem to know both the energy and the time of the photon at the same time. But, as the great physicist Niels Bohr pointed out, Einstein overlooked the fact that when the photon would be released, there would be a recoil, creating uncertainty in the position of the box in earth’s gravitational field, which (ironically according to Einstein’s own theory of General Relativity) would create further uncertainty in the time recorded, since time slows down due to spacetime curvature, which depends on matter-energy, which gives rise to the illusion of gravity. Thus, the uncertainty principle is accurate. Nevertheless, Einstein would later challenge the uncertainty principle and question the completeness of quantum mechanics in the EPR paper. Not even Bohr could find a satisfactory solution this time. The EPR “paradox” remains one of the greatest mysteries in science.

**The EPR Argument**

The original EPR argument[3] was this: If one measured the position of one of two coherent electrons, that is, electrons belonging to the same quantum system, the wave function for the entire system would collapse into one of its position eigenstates, instantly revealing the other electron’s position. Measuring one electron’s momentum, similarly, would reveal the other electron’s momentum, meaning the other electron “should have both these quantities ready”, that is, existing at the same time, violating the uncertainty principle. This is because the second electron cannot possibly know in advance which quantity among position and momentum the person may choose to measure.

There is another form of the argument. Two coherent electrons must have opposite spin[4]. (One can also understand it this way: two similar particles whose combined spin is zero, are used. Thus, they must have opposite spin.) If such an entangled pair of electrons is released from a common source in opposite directions at near-light velocities, over time the electrons may reach opposite ends of the universe. But as per quantum mechanics, we know that the spins of the electrons are undefined before they are observed. If an electron is observed when it is already far away from the other, say light-years away, its spin will collapse from a superposition of up and down states to a distinct state. At that instant, the other electron’s spin must also, without direct observation, collapse to the opposite spin. Something passes between the electrons faster than light, violating causality.

*Figure 1: An image depicting quantum entanglement.*

Indeed, things can remain entangled through space and time. Many physicists assumed that there were some “hidden variables” yet to be discovered, and once they were discovered, all the bizarre ideas of quantum mechanics, when interpreted along with that, would become “acceptable” again. The idea was that local, hidden variables already carried the information about the particles (say, the spin).

But John Bell has mathematically proved that there can be no hidden theory compatible with quantum theory. Bell’s theorem[5] has been repeatedly verified experimentally, ruling out the possibility of faulty instruments, and thus, faulty results. As shown through the use of photons, things separated through spacetime can, in fact, remain entangled. Nature is non-local.

According to Einstein, Podolsky, and Rosen (EPR), “A sufficient condition for the reality of a physical quantity is the possibility of predicting it with certainty, without, in any manner, disturbing the system.”[6] Niels Bohr, who has already refuted Einstein’s previous arguments against quantum mechanics, pointed out that not disturbing the system is in itself impossible due to the inevitable interaction (in light of quantum mechanics) between the objects and the measuring instruments. In fact, the act of measurement is fundamentally crucial. It can even change the properties of the object being measured.

To derive his inequality, Bell used some general and logical facts with which everyone could agree, except for Einstein’s condition of locality, which he assumed to be true. If experiments showed that the inequality was violated, this would mean that one of the premises in his derivation was false. Bell interpreted this to mean that nature was non-local.

Experiments (by John Clauser, Alain Aspect, et al.[7]) using photons, polarization filters, PMTs (Photomultiplier Tube), etc… indicated experimental verification of the *violation* of Bell’s Inequality. This has been argued to mean that in spite of the local appearances of phenomena, our world is actually supported by an invisible reality that is unmediated and allows instantaneous, faster-than-light communications. These interactions do not diminish with distance and can link up locations without crossing space. Though it may seem so,there is nothing “spooky” with the concept. We – accustomed to classical mechanics – find it bizarre. The key is that *entangled systems*, no matter how far apart, *are described by a single wave-function*[8] in quantum mechanics. But it must also be noted that the results of these experiments are not 100% percent foolproof. For one, these experiments are based on the statistical analysis of hundreds of measurements.

Furthermore, we must not forget that it is on Newtonian mechanics, that the foundations of physics rest on. If Newton were not to form his notions of reality, we could not have dreamt of correcting them.

**The Advent of Quantum Mechanics**

Science has grown strange from the days of Sir Issac Newton. It was from time immemorial, assumed that the best way to understand the world is to focus on the pieces, and figure out how these pieces fit together. This approach is known as reductionism. But the quantum reality we live in cannot be understood that way. The reality as described by classical physics is only a rough approximation of the actual reality. Even quantum field theory is supposed to be a low-energy approximation of a deeper unified theory that we are yet to discover. At much higher energies, all the forces are unified. In light of quantum mechanics, can we be certain about anything after all? From transistors to particle acceleration to Scanning Tunneling Microscopy to electron microscopy to quantum computing, quantum mechanics is used everywhere, accepted widely, and verified theoretically. Yet, it challenges the basic notions of the nature of reality. Quantum mechanics even leaves open the exciting possibility of human teleportation, that is, instantaneously transporting humans over astronomical distances. Moreover, Parapsychologists, like Dean Radin[9], claim that quantum mechanics can solve the mysteries of ESP (Extra Sensory Perception).

Quantum mechanics may be observer-dependent. If this is true, there can be no single version of reality. The very fact that humans enjoy so complex a life is open testimony to that. (However, this is an extremely sensitive topic, and is prone to be misunderstood. As of yet, the debate between realism and solipsism hasn’t been conclusively settled. Maybe the world exists independent of our observations; maybe observation matters and the reality is subjective.)

Quantum mechanics is more than mathematical manipulations to get a desired result, or a model to answer certain questions – rather a revolutionary set of ideas that have the potential to philosophically address the ultimate and most fundamental purpose of science. Mathematics can show the way, but it is the science that can actually take us forward through this way. We will go on creating theories each more accurate than the last, but a final unified field theory, in its true sense, may be forever beyond our reach. Interestingly, it may also be that there is no final theory. Theories will be more and more fundamental than previous theories, but we can reach no final explanation which is not in need of further explanation.

**Einstein, Relativity and Cosmology**

A very interesting figure during the quantum revolution was perhaps the best man during the marriage of the law of constancy of the speed of light in a vacuum with the principle of relativity in the restricted sense. The special theory of relativity brought with it a hitherto-unknown, young clerk into the physics community: Albert Einstein.

Einstein proposed to treat time as the fourth dimension, which, along with the three space dimensions, formed a spacetime continuum. Using the Lorentz transformations, he was able to show that perceptions in length and time changes, according to an external observer who is moving at a velocity different from that of the vessel being observed. He predicted that no body, with no matter how small a rest mass, could ever reach or exceed the speed of light. Maxwell had already proved that light is an electromagnetic wave, or a wave with vibrating electric and magnetic fields, that traveled at the constant speed of 299792458 meters per second (in a vacuum). Based on this result, Einstein predicted modifications to the structure of spacetime. Another result of the special theory of relativity was that mass and energy can be converted into one another, and are related by the famous equation . (Since the speed of light, c is a huge number, it is obvious that even a small amount of mass can give rise to huge amounts of energy. This is the principle behind nuclear bombs and power plants.) Further, the entire mass-energy-momentum relation is given by . Taking into account the change in mass due to velocity, a special, dimensionless term ‘gamma’ is multiplied with the rest mass of a body, to get the relativistic mass. The total energy, thus, is given by .

Special relativity could also provide an explanation to Olbers’ paradox, which states that it is paradoxical for the sky to be dark at night since, in an infinite universe, there should be an infinite number of stars with their collective light brightening the sky. It was incorrectly assumed that this is because the light from the stars is blocked by the dust. If this was so, then due to the light, this dust would eventually also heat up and glow. Another proposed solution held that the tremendous redshift of distant galaxies, i.e., the lengthening of the wavelength of light they emit due to the expansion of the universe, would move light out of the visible range into the invisible infrared. But if this explanation were true, shorter-wavelength ultraviolet light would also be shifted into the visible range and we could see them, which does not happen. The actual solution is that light from all the distant stars has not yet been able to reach us and what we see is actually the past. This is because light travels at a great but finite speed and this speed is not too great when compared to the expansion of the universe. This is also complemented by the fact that many older stars eventually die out.

Another interesting fact is that, though the universe is about 13.77 billion years old, the observable radius of the universe is about 46 billion light-years, i.e., we can see up to 46 billion light-years in any direction. This means the diameter of the visible universe is roughly 92 billion light-years. Assuming the universe expanded at light speed, how can it still end up being larger than 13.77 billion light-years? This is because of the inflationary phase. Though nothing can exceed light speed, space itself can. A good analogy is the fact that when a balloon with a lot of spots on it is inflated, though the spots do not move by themselves, the distance between them keeps increasing. Also, the universe is constantly expanding in *all* directions. Thus, the source of light is constantly moving away from us too and we are seeing the light it emitted a long time ago. The light it emitted has to constantly travel more and more distance to reach us, due to the expansion.

But Einstein was not the type to be satisfied by the special theory of relativity. What about perceptions of length and time between two bodies with a non-uniform, that is, accelerating relative motion? What about stars and other astronomical phenomena? If the physics community was shocked and impressed by the clerk’s special theory of relativity, they were knocked out by the general theory of relativity. After over ten years of struggle through the thickets of spacetime and matter-energy, Einstein wrote down the field equations for gravity. This is perhaps the best example of a theory comparable to quantum mechanics, yet one that has been developed almost single-handedly. General Relativity provided a new interpretation of spacetime, forever transforming the name “Einstein” into a synonym for “genius”. General relativity, like quantum mechanics, remains one of the greatest theories in physics, and experimentally tested as well. It has predicted the existence of black holes, dark energy, gravitational waves etc.

Einstein’s philosophy was to explain nature in terms of geometry. At that time, few people realized the potential of Einstein’s relativity theory, and in fact, he was awarded the 1921 Nobel Prize in Physics mainly for his interpretation of the photoelectric effect (which seems child’s play when compared to relativity) and “for his services to theoretical physics”. It is truly wonderful to picture the walrus-mustached young man imagining himself to be in a spaceship moving at light speed and playing with light – bouncing it up and down so that if viewed from outside, the horizontal movement of the spaceship combined with the movement of light produces an effect that light travels a greater distance…

Amid the chaos of the second world war and the then-new quantum mechanics “heresy”, Einstein would remain engaged in a futile attempt to unify gravitation with electromagnetism, using geometry alone. He would keep this up for over thirty years until his death. He could not bring himself to truly accept the strange, bizarre ideas of quantum mechanics. He believed in a perfectly-ordered and deterministic universe, putting up his famous mantra against quantum randomness – “*God does not play dice with the world.*” God must, according to him, follow uniform logic and reason, as per which the universe functions. Niels Bohr, however, retorted with “*Stop telling God what to do!*”[10]

One single thought, that of chasing a light beam, transformed a young student worried over petty matters – into a revolutionary physicist who, had he not chosen to keep his eyes shut to quantum mechanics, probably would have come closer than anybody to the ultimate truth (if any). Even today it is surprising how that young man took to his shoulders the insurmountable task of writing the equations for curved spacetime and the behavior of matter-energy. In light of all he has done, it is indeed obvious why the term “genius” is inextricably linked with the wild-haired, casually-dressed Princeton physicist.

Yet, in light of quantum mechanics, Einstein’s field equations seem to have lost some of its charm. Relativity only modified classical mechanics, and these modifications become of any practical importance only when near-light velocities are considered. On the other hand, overthrowing the orderly world of Sir Issac Newton, quantum mechanics, though accurately describing three of four fundamental forces of nature, makes radical demands on classical physics and the common-sense notions about the very nature of reality itself, that too about the omnipresent atoms we are made of.

Experiments on black body radiation[11], for the first time, revealed the incompleteness of classical theories. Thus, the need for an entirely new concept was felt. That turned out to be the quantization of energy into discrete packets called quanta. Before that, it was believed that energy was directly proportional to intensity, and a body continuously absorbs/emits energy. But this cannot explain the fact that on heating a body, the color changes, and not only the intensity of the color. Thus, the new assumption was that energy is directly proportional to frequency, and is related to frequency by Plack’s constant , that is, .

Thus, with Max Planck’s concept of quanta, quantum theory was born. At the beginning of the twentieth century, few people realized that quantum theory would evolve to be a theory so powerful that it would attempt to reach the fundamental nature of reality itself; it would give rise to earth-shaking technologies as well as earth-shaking paradoxes; it would give birth to a Grand Unified Theory that would come tantalizingly close to ultimate unification of the four fundamental forces of nature, and finally, string theory, which may indeed, after all, achieve Einstein’s and the physics community’s long-desired unification. Quantum theory has changed science, redefined everything fundamentally – it is not merely a theory, but a scientific revolution.

The journey of quantum physics has been a long and inspiring one. Today, researchers suggest that quantum entanglement may even be the reason behind thermalization[12], or the phenomena of hot things gradually cooling down with time (and never the reverse). It is reasonable to assume that gradually, quantum physics will influence all of science. Every modern theory must be in accordance with the laws of quantum physics to be acceptable. The question regarding quantum mechanics is no more “if”, but “when”*.*

**Quantum Randomness and Schrödinger’s Cat**

Another interesting name to be considered is Erwin Schrödinger, the great Austrian physicist who developed the Schrödinger wave equation, a pivotal equation of unimaginable importance to modern quantum mechanics. He, like Einstein (they were actually friends), did not truly accept quantum mechanics and even regretted discovering the equation that brought him a Nobel Prize in Physics. But once more, the battle against quantum mechanics remained futile – this new revolution was not to be easily disproved.

Unlike Heisenberg, Bohr, Born, and others, Einstein and Schrödinger, two exceptionally-talented physicists, failed to accept the bizarre ideas of duality, non-locality, nether-states, and uncertainty, which are inherent in quantum mechanics. While Einstein questioned the completeness of the quantum mechanical description of reality; Schrödinger devised his famous cat paradox. If a cat is kept in a box under conditions such that it has a 50% chance of living and the same of dying, and this was made solely dependent on some other system (say, a radioactive substance attached to a Geiger counter such that if it decays, the counter will trigger a hammer to break a poison bottle and kill the cat and otherwise not) that is probabilistic, then the quantum states of the cat and this system become entangled. When you take into account the characteristics of radioactive decay, quantum mechanics predict that the cat must both live and die at the same time, before it is observed. (A similar thought experiment emphasizes on gunpowder exploding and not exploding at the same time.) This only shows that either the other possibilities that are not fulfilled in this universe, are fulfilled elsewhere, in a parallel universe (Hugh Everett’s many-worlds interpretation), or direct observation is required to collapse the cat’s wave-function from a superposition of states to a distinct state (Niels Bohr’s Copenhagen interpretation). The cat may itself perform this act of observation. Maybe the intensity of the observation matters, otherwise one could say that the cat, initially capable of observation, must always live in the above thought experiment. Also, it is important to note that, in this thought experiment, the cat should never be allowed to physically interact with the system of the radioactive substance and Geiger counter connected with the hammer and poison bottle.

*Figure 2: A figure depicting the Schrödinger’s cat thought experiment.*

All of this raises questions on the existence of a supreme observer (God) and other philosophical issues. (As of yet, there is no widely-accepted conclusion as to whether the wave-function has any physical significance.) The point is that, if an external observer is required to collapse a cat’s wave-function, then a second observer must be responsible for the existence of the cat’s observer. We need a third observer for the second and so on till we get infinitely-many observers. Eugene Wigner interpreted this to mean that there must be a supreme observer (God) who is keeping watch over all of us. However, there are many other, perhaps even more appropriate ways of looking at the situation. This also triggered newer interpretations of observation in quantum mechanics, like the Hofmann-Patekar theory[13]. The Hofmann-Patekar paper approaches the problem as described: Say the cat is still in the box, but rather than looking inside to determine whether the cat is alive or dead, you set up a camera outside the box that can somehow take a picture inside of it (for the sake of the thought experiment, ignore the fact that physical cameras do not actually work like that). Once the picture is taken, the camera has two kinds of information: how the cat changed as a result of the picture being taken (what the researchers call a quantum tag) and whether the cat is alive or dead after the interaction. None of that information has been lost yet. And depending on how you choose to “develop” the image, you retrieve one or the other piece of information.

The resolution refers to how much information is extracted from the quantum system, and the disturbance refers to how much the system is irreversibly changed.

“What I found surprising is that the ability to undo the disturbance is directly related to how much information you get about the observable, or the physical quantity they are measuring”, Hofmann said. “The mathematics is pretty exact here.”

By doing so, Hofmann and Patekar are able to assume that all the photons involved in the initial interaction (or peek at the cat) are captured without losing any information about the cat’s state. So, before the readout, everything there is to know about the cat’s state (and about and how looking at it changed it) is still available. It is only when we read out the information that we lose some of it.

The Copenhagen interpretation of Schrödinger’s cat thought experiment suggested that it is observation that collapses the wave-function of a particle from the superposition of all possible states to a single, distinct state. But does it give a satisfactory explanation as to why a particular state, among all the possible states, should be observed? The entire thought experiment is based on entanglement, superposition, and measurement[14]. Entanglement implies that an entirely new and single quantum system has formed. In fact, it follows that the existence of only one thing is NOT random, and that is the wave-function of the entire universe as a whole. We may evolve the wave-functions of any system according to Schrödinger’s equation. But in the end, all it gives us is probability. And research in modern science seems to suggest, more and more, that the universe is indeed random. In fact, it is precisely because the universe is random and because of entropic increases that the second law of thermodynamics holds that life, due to some lucky accidents, ever formed in our universe. Entropy is equivalent to information. In fact, it may be that quantum measurements and the expanding universe are the main reasons that entropy is increasing. But it goes deeper than that. Fundamentally, it must be the case that the concept of conservation applies to probability as well. As soon as we open the box containing Schrödinger’s cat and peek into it, we ourselves get entangled with the system. If we see the cat alive here, it means that different versions of us, in a parallel universe, see the cat dead and so on. All possible states must manifest itself. It follows that, constantly with the evolution of time, new worlds are being created, branching off… This sounds crazy. Thus, it may even be the case that an infinite number of universes are present, and the infinite possibilities take place in them. Indeed, once the concept of infinity comes in, it is likely that we might never understand it in its entirety. But this is another way of looking at the cat, and maybe a better one. Also, it is important to note that, in this thought experiment, the cat should never be allowed to physically interact with the system of the radioactive substance and Geiger counter connected with the hammer and poison bottle.

Among the many philosophical interpretations of what quantum mechanics[15] is telling us about the fundamental nature of reality, the most notable ones include the Copenhagen interpretation, the transactional interpretation, the many-worlds interpretation etc..

According to the Copenhagen interpretation, nature is intrinsically probabilistic. In his Copenhagen interpretation, Bohr could never specify exactly what counted as observation. For instance, the wave-function can interact with detectors without collapsing. So maybe simply recording or keeping track of a particle’s properties does not count as observation. We need conscious beings to do the observation. But then, what is consciousness? How can we come up with a rigorous definition of consciousness?

To resolve this problem, some have suggested that we are living in a computer simulation. The computer does not calculate the exact properties of a particle until it is relevant to and observed by the player, simply to save energy. For instance, in video games, details of the surroundings become prominent only on zooming in. Just like that, only on direct observation are we aware of the exact properties of a particle, not before that.

However, the question remains how exactly the wave-function collapses. Objective collapse theories propose that wave-functions have a small chance of collapsing on their own at each moment, without observation. If there are many entangled particles in a system, then the chance of collapse increases – since if a single particle collapses to a particular state, the other entangled particles, and as a result the entire system, will collapse to that same state. Others believe that interaction with spacetime curvature causes the collapse of the wave-function. The latter hypothesis is, fortunately, testable.

Now let us consider another deep question. When a wave-function collapses, it must collapse everywhere simultaneously. This means that it is possible, under certain circumstances, for information to travel at a speed greater than the speed of light, as we saw in the EPR argument. (The idea of pilot waves, which guide the motion of particles in spacetime, face the same problem – interactions at any one location must be felt everywhere instantly, violating relativity.) This means, theoretically, we could send information back in time. This is the essence of the EPR argument. But the idea of retro-causality views all these from a different perspective. It is indeed not possible to travel faster than light in space. But particles may travel backward in time. Indeed, a particle traveling forward in time is indistinguishable from its antiparticle travelling backward in time. Two particles may communicate with each other through time, not space. This idea might seem appealing to many, and it has its advantages. But, as always, problems remain.

According to the transactional interpretation, the wave-function moves forward in time, while its complex conjugate moves backward in time. It is the transaction between these two wave-functions that lets the universe decide which path to select, independent of any observer. Does this mean that the future is predetermined? No. The wave-functions travelling backward in time are travelling from possible futures rather than a future which is completely predetermined.

Speaking of determinism, the concept of super-determinism holds that everything was always predetermined, and this information was transmitted to all the particles during the Big Bang. In contrast, according to quantum Bayesianism (QBism), the universe is intrinsically probabilistic. There is no objective reality, only personal objective beliefs. The wave-function represents our personal subjective beliefs about the objective reality. There is, thus, no objective wave-function. The collapse of the wave-function is simply us updating our beliefs about future probabilities.

Other interpretations like the relational interpretation proposes that any system, whether it is conscious or not, can observe. Observers may disagree whether the wave-function has collapsed or not, just like in relativity, observers in different reference bodies may disagree about the time when an event took place. Consider the EPR thought experiment. When the particles have reached far apart, no observer can actually measure both spins simultaneously. From the perspective of the individual observer, the spin of the unobserved particle is still undefined, until the observer interacts with something that has measured the spin of that particle.

Indeed, it is a strange new world. Rules of known logic simply fail. Some people have argued that known logic is just a special case of a more fundamental logic – quantum logic. Our ancestors used known logic. They did not need to use quantum logic at any point. So, we are hardwired to believe in known logic. However, when we have reached as far as logic, it would be wise to stop – else we will soon be questioning whether we exist at all!

One of the greatest misconceptions regarding quantum mechanics is that it is the science of only the subatomic world. Many people divide physics into three parts – classical mechanics (which describes the macroscopic world); relativistic mechanics (which describes mechanics of particles moving at near-light velocities) and quantum mechanics (which describes the microscopic, subatomic world). We often forget that classical mechanics is only a rough approximation of quantum mechanics. The basic principles of quantum mechanics are applicable even to the macroscopic world, and classical mechanics only produces an approximation of the results obtained by using quantum mechanics. For example, we do not perceive the wave nature of a macroscopic object since the associated wavelength is so small (due to the large mass) that we cannot detect and perceive it. But wave-particle duality holds for macroscopic objects just as well as microscopic objects.

**Physics Today**

Today, it seems reasonable that the universe is indeed random. Nature is ruled by probability and simply tries out all possible paths. But let us not forget that, according to many renowned scientists and philosophers, the world we see around us is emergent from a deeper structure, of which we have little or no understanding.

Erwin Schrödinger, so many years back, had already formed the then-unusual idea that life was both orderly and complex. He saw aperiodicity as the source of life’s special qualities. (His ideas on life and biology also inspired Watson and Crick’s work on DNA[16].) Perhaps he, a great physicist and philosopher, gained insight that one day, quantum chaos would link classical mechanics and quantum mechanics using chaos theory, and this might lead to our understanding of the fundamental nature of reality itself.

The focus now is to find a unified field theory that would unite the four forces of nature. It has been particularly difficult to unite gravity with electromagnetic, weak nuclear, and strong nuclear forces. The idea of unification is a fundamental philosophy in physics. For instance, with the discovery of atoms, ice, water, and water vapor have been, in some sense, unified into water. All three are made up of molecules. Electricity and magnetism have been unified into electromagnetic interaction, which with weak interaction, has been recently unified into electroweak interaction, which, with strong interaction, gives rise to grand unified interaction (described by the Standard Model or/and Grand Unified Theory). If we ever succeed in unifying this with gravitational interaction, which is still subject to a lot of controversies, we will achieve a universal unification of the four fundamental forces of nature. Then, we could, in a single theoretical picture, describe all the ways in which particles interact with one another.

A candidate for the theory of “everything”, string theory proposes fundamental “strings”, the different modes of vibrations on which corresponds to so many particles, are the most fundamental “building blocks of the universe”, and no particle is any more fundamental than any other particle. String theory only “works” in 10 dimensions. It may be true that there are more dimensions than we can perceive, some curled up to the Planck length. From the Kaluza-Klein concept, we see that increasing the number of dimensions helps to “accommodate” more forces. String theory essentially replaces the idea of a zero-dimensional point particle with the idea of a one-dimensional string of energy. It is a promising approach to reconcile quantum mechanics with general relativity. In string theory, the approach is to incorporate gravity to quantum mechanics, while another theory, Loop Quantum Gravity, attempts to apply the principles of quantum mechanics to gravity. General relativity describes gravity as a *manifestation of geometry in spacetime*. Loop Quantum Gravity suggests that space itself is discrete, quantized, and granular (not continuous), just like quantum mechanics proved that energy is discrete. In this view, space is a set of finite loops intertwined in a network of loops known as a spin network. Both approaches have their advantages and drawbacks. There is much scope for exploration in these areas.

Michio Kaku beautifully summarizes the basic idea of string theory in these lines: “In string theory, all particles are vibrations on a tiny rubber band; physics is the harmonies on the string; chemistry is the melodies we play on vibrating strings; the universe is a symphony of strings, and the ‘Mind of God’ is cosmic music resonating in 11-dimensional hyperspace..”[17]

Today, researchers make a radical claim. Gravity is not a fundamental force. Thus, when we try to unify it with quantum field theory, paradoxes and mathematical inconsistencies arise. Quantum field theory describes forces as taking place due to the exchange of particles called bosons. However, gravity is a different story altogether. It is described by the geometry of spacetime, and *not* by the exchange of any particle. However, though some have theorized the existence of the so-called graviton (or the boson responsible for gravitational force), no experimental data confirms this, and for all we know, might never do so.

Erik Verlinde[18] argued that gravity can be explained as an entropic force. He proposed to interpret the force in Newton’s second law and gravity as entropic forces. An entropic force in a system is a force that results from the entire system’s thermodynamic tendency to increase its entropy, rather than from a particular underlying microscopic force. The idea is that gravity is not a fundamental interaction, but rather emerges from the universe trying to maximize disorder. Gravity is therefore an entropic force – a natural consequence of thermodynamics.

Modern cosmology states that the universe is accelerating and expanding continuously and therefore has its entropy being increased at every given moment. More the expansion, increasing is the disorder, the information contained, and the number of ways a particular arrangement can be achieved. And so, entropy is increasing too, at a tremendous rate. During this expansion, some particles may stumble upon one another and will have gravitational attraction between them, thus, for a certain time period, establishing stable behavior. Galaxies are formed in those regions where the *regional* density is high. Entropy may not increase at that particular portion of space, but it is at the expense of an increase in entropy in its surroundings so that the net entropy still increases. Moreover, as the universe is expanding, the effective gravity between galaxies moving apart is decreasing (in accordance with Newton’s law of gravitation) because of an increase in the distances among them.

This new interpretation of gravity might give us new insights into the information paradox. According to quantum mechanics, the quantum information associated with a particle can never be completely destroyed. It is also known that once a particle crosses the event horizon of a black hole, no further information associated with that particle can be extracted. As was earlier assumed, if black holes are eternal, then it can be the case that a particle’s information is stored inside the black hole, somehow. However, we must take into account the phenomena of pair production, or the materialization of a particle and its oppositely-charged partner, called antiparticle, in a vacuum; and annihilation, or the contact, explosion, and vanishing of the particle-antiparticle pairs that pop up in a vacuum, to conserve energy. Stephen Hawking proved that if pair production takes place near the event horizon of a black hole, then before annihilation can occur, the partner of the particle will be sucked into the black hole. Thus, to account for the existence of the particle which is not annihilated, the black hole must give up part of its energy and evaporate gradually. This gave rise to the paradox about the information that was supposed to be preserved inside the black hole. To resolve this paradox, many ideas have been suggested. Notable among them is the holographic principle. In this view, the universe is in fact two-dimensional, with the information of all the particles in it projected like a hologram, to create the illusion of a three-dimensional universe. The information of the particle is simply etched onto the surface of the black hole.

All electric and magnetic fields fluctuate constantly, even when electromagnetic waves are absent. These fluctuations induce the apparently-spontaneous emission of photons by atoms in excited states. The vacuum fluctuations can be regarded as a sea of virtual photons, so short-lived that they do not violate energy conservation because of the uncertainty principle. These photons give rise to the Casimir effect. Only virtual photons with certain specific wavelengths can be reflected back-and-forth between two parallel metal plates, whereas outside the plates, virtual photons of all wavelengths can be reflected by them. The result is a tiny, but detectable force that tends to push the plates together, even in empty space. However, this effect cannot be exploited to generate practical amounts of energy.

Interestingly, the idea of production and annihilation of particle-antiparticle pairs in a vacuum, though allowed for a limited time by the uncertainty principle, is actually one of the greatest scandals in physics. However, ultimately it is vacuum energy that remains one of the greatest catastrophes in physics[19]. The measured value of vacuum energy is less than the predicted value by a factor of 10^{120}. (It has been proposed that the ‘missing value’ might actually ‘leak out’ to a parallel universe through a wormhole.)

The origin of matter and its creation, interestingly, is still one of the more largely debated topics in physics and a great mystery that still astounds scientists today. The baryon asymmetry problem, also known as the matter-antimatter asymmetry problem, calls attention to the fact that there are not equal amounts of matter and antimatter in the universe. Both the theory of relativity and the Standard Model of particle physics suggest that the Big Bang should have produced equal amounts of matter and antimatter, but it is clearly evident today that there is substantially more matter that exists. This baffling phenomenon has led some scientists to suggest that there may be in existence, an antimatter universe that extends backward in time beyond the big bang, and our universe is simply a mirror image of that[20].

It is indeed puzzling that the laws of physics make no distinction between the past and the future, meaning it is possible for time to ‘run backward’. But we do not see a broken cup pick itself up from the floor and repair itself. This leaves open the possibility of a CPT (Charge-Parity-Time)-reversed universe, for if all of these are reversed, the laws of physics simply do not change. (In a CPT-reversed universe, positive and negative, left and right and past and future would be inverted.) Boltzmann assumed that our universe began in a very unlikely state. Entropy increases both into the past and the future, and we are, thus, existing on the vertex of a lower ‘V’ shape from the original equilibrium state, in an entropy-time graph. Our universe, in this view, could’ve come into existence just a moment ago, complete with traces of the past of a greater universe. In statistical mechanics, there is a small but non-zero chance that some gas molecules in an *open* container do *not* come out of it, even when the surroundings are devoid of any gas molecules. This does not generally happen, for everything will tend to achieve an equilibrium state. But if we wait long enough, at one time such things must occur for there is still a probability for them to occur, no matter how small. Given enough time, we may see some atoms in a system by themselves huddle in a corner of the system, and not move. In such cases, as the system was more in equilibrium *before *the unlikely state was achieved, entropy also increases into the past.

*Figure 3: A figure explaining some fundamental approaches in modern physics.*

Although physicist Albert A. Michelson once said, “All that remains in physics is to fill in the sixth decimal place”, this is actually false (he himself regretted saying this later). Even if we do discover a true unified field theory, it would only open up new areas of research. The ultimate goal of physics cannot be to find a unified field theory; that can only take us a step forward in our quest to understand the fundamental nature of reality. As string theorist Michio Kaku says, “Far from the end, it is only the beginning.”[21]

**Conclusion **

Humans have always strived to make sense of the world around them. We have strived to discover universal laws that apply to a large range of phenomena. For this, we need a fundamental science, one that does not depend on many presuppositions. From Newton’s days to the present, physics has built up our view of the universe, from scratch, using the convenient language of mathematics. A natural selection of ideas has helped us filter incorrect and useless theories and move toward a universal theory. In the old days, scientists were driven by the urge to understand the endless cosmos. After Newton’s revolutionary and brilliant insights, the scientific community was almost certain of understanding, with a hundred percent certainty, every phenomenon that goes on in a universe of which they form a very insignificant part. Scientists were certain that if they went on dividing matter, there would soon come a time when what is left of that matter cannot be divided further. However, today we know that this approach was incorrect. Even after discovering subatomic particles and proposing models about the internal structure of the atom, we realized that all those particles are not point particles, and they cannot be treated as a microscopic sphere, whose position and momentum can certainly be known. Rather, as Schrödinger put it, “What we observe as material bodies and forces are nothing but shapes and variations in the structure of space.”[22]

Today, physics does not work on the approach of reductionism. Rather, the approach of unification is more preferable. Particles and waves have, in some sense, been unified. Particles are nothing but the results of vibrations of their respective fields, according to quantum field theory. Physicists want universal laws that can explain every possible interaction between particles, in a single theoretical framework. Physicists like Einstein believe in an isolated God who had created the universe and set the laws according to which it functions but does not interfere in our lives. Not even God can change or tamper with those laws. For, if we assume that He can, there would be no point to science. We must accept such an explanation only after exhausting all other scientific possibilities.

The effort to achieve unification in physics has given rise to many new theories, interpretations, insights as well as problems. It is clear that grand unification cannot be the ultimate target of physics, for that would only open up newer scopes of research. Just like Gödel proved that mathematics is inexhaustible, in all probabilities, physics might be too. No matter how far we advance, physics will never fail to surprise and inspire us. The journey from Newtonian mechanics to quantum mechanics is enough to show us that we should keep a flexible and open mind, since in physics, anything may be possible!

Let us just wonder, for a moment, the journey of physics, from the laws of motion and thermodynamics to quantum mechanics and string theory… Where are we bound to reach? How much more strange can nature get? It is truly amazing and rather unnerving to comprehend that the vast and endless universe we live in, is filled with things as unfathomable as virtual particles and black holes. We have come a long way, from grasping why the sky looks black at night to finding the Higgs boson. But the journey is, by no means, over. It may have only just begun…

To quote the great physicist Stephen Hawking, “… if we do discover a complete theory, it should in time be understandable in broad principle by everyone, not just a few scientists. Then we shall all, philosophers, scientists, and just ordinary people, be able to take part in the discussion of the question of why it is that we and the universe exist. If we find the answer to that, it would be the ultimate triumph of human reason – for then we would know the mind of God.”[23]

**References**

[1] Daly, Mary. “Bentley’s Paradox”. 2010. https://marydaly.wordpress.com/2010/06/26/bentleys-paradox/

[2] The Editors of Encyclopaedia Britannica. “Uncertainty principle”. 1998. https://www.britannica.com/science/uncertainty-principle

[3] Jones, Andrew Zimmerman. “EPR Paradox in Physics”. 2019. https://www.thoughtco.com/epr-paradox-in-physics-2699186

[4] Tavel, Morton, Kurt T. Bachmann, Victor J. Stenger. “What exactly is the ‘spin’ of subatomic particles such as electrons and protons? Does it have any physical significance, analogous to the spin of a planet?”. 1999. https://www.scientificamerican.com/article/what-exactly-is-the-spin/

[5] Bhatia, Aatish. “The Experiment That Forever Changed How We Think About Reality”. 2014. https://www.wired.com/2014/01/bells-theorem/

[6] Einstein, Albert, Boris Podolsky, Nathan Rosen. “Can Quantum-Mechanical Description of Physical Reality Be Considered Complete?”. 1935. https://journals.aps.org/pr/pdf/10.1103/PhysRev.47.777

[7] Aspect, Alain. “Bell’s Theorem: The Naive View Of An Experimentalist”. 2004. https://arxiv.org/ftp/quant-ph/papers/0402/0402001.pdf

[8] Helmenstine, Anne Marie. “What is a Wave Function?”. 2019. https://www.thoughtco.com/definition-of-wavefunction-605790

[9] Radin, Dean. “Entangled Minds: Extrasensory Experiences in a Quantum Reality”. 2006. Paraview/Pocket Books.

[10] Einstein and Bohr, The Center for History of Physics, history.aip.org/history/exhibits/einstein/ae63.htm.

[11] Chemistry LibreTexts. “Blackbody Radiation”. 2020. https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supplemental_Modules_(Physical_and_Theoretical_Chemistry)/Quantum_Mechanics/01._Waves_and_Particles/Blackbody_Radiation#:~:text=Blackbody%20radiation%20is%20a%20theoretical,absorbs%20all%20frequencies%20of%20light.&text=As%20the%20temperature%20increases%2C%20the,the%20area%20under%20the%20curve.

[12] Malewar, Amit. “Linking quantum entanglement to cold coffee”. 2020. https://www.techexplorist.com/linking-quantum-entanglement-cold-coffee/29659/

[13] Najjar, Dana. “Physicists Can Finally Peek At Schrödinger’s Cat Without Killing It Forever”. 2019. https://www.livescience.com/schrodingers-cat-can-be-peeked-at.html

[14] Muller, Derek. “Parallel Worlds Probably Exist. Here’s Why”. 2020. https://youtu.be/kTXTPe3wahc

[15] Khutoryansky, Eugene. “Quantum Physics – list of Philosophical Interpretations”. 2020. https://youtu.be/XQ25E9gu4qI

[16] Brooks, Michael. “Erwin Schrödinger: A Misunderstood Icon”. https://www.the-tls.co.uk/articles/erwin-schrodinger-misunderstood-icon/

[17] Michio Kaku Quotes. BrainyQuote.com. 2020. https://www.brainyquote.com/quotes/michio_kaku_615171

[18] Ratner, Paul. “Physicist advances a radical theory of gravity”. 2019. https://bigthink.com/surprising-science/physicist-radical-theory-of-gravity?rebelltitem=1#rebelltitem1

[19] Sorna, Anumeena. “The Nature of Nothingness: Understanding the Vacuum Catastrophe”. 2018. https://medium.com/nakshatra/the-nature-of-nothingness-understanding-the-vacuum-catastrophe-c04033e752f4

[20] Siegel, Ethan. “This Is The One Symmetry That The Universe Must Never Violate”. 2020. https://www.forbes.com/sites/startswithabang/2020/03/25/the-one-symmetry-that-the-universe-forbids-us-from-violating/#66c5dd0a2d4f

[21] Kaku, Michio. “Beyond Einstein: The Cosmic Quest for the Theory of the Universe”. 1987. Bantam.

[22] Erwin Schrodinger Quotes. BrainyQuote.com. 2020. https://www.brainyquote.com/quotes/erwin_schrodinger_304801

[23] Hawking, Stephen. “A Brief History of Time”. 1988. Bantam.

**Figure References**

Figure 1: EPR Paradox in Physics: Definition and Explanation. ThoughtCo. Accessed 28 August 2020. https://www.thoughtco.com/thmb/PJWuBLEpSk2Gh6NacYSjRbJIMQI=/2080×1441/filters:fill(auto,1)/GettyImages-623682717-57dd5b693df78c9ccef52b71.jpg

Figure 2: Why is Schrodinger’s cat both dead and alive? Is this not a paradox? Ask a Mathematician. Accessed 28 August 2020. https://www.askamathematician.com/wp-content/uploads/2013/04/cat.jpg

Figure 3: Theoretical physics: The origins of space and time. Nature. Accessed 28 August 2020. https://www.nature.com/news/polopoly_fs/7.12027.1378900008!/image/quantum-gravity-nature-online.jpg_gen/derivatives/landscape_630/quantum-gravity-nature-online.jpg

### Biography

#### Arpan Dey

Arpan Dey is a high school student from India. He is interested in physics and mathematics. He wishes to pursue quantum mechanics and/or nonlinear dynamics in the future. He is also an aviation enthusiast.

#### Sonnet Xu

Sonnet Xu is a rising sophomore from Michigan. She loves robotics, STEM, AI, physics, and computer science. She loves biking and reading in her free time. You can often catch her running around her neighborhood!